The present invention relates to an image recording method for an optical scan type electrophotographic recording apparatus and, further, to an image recording method for an optical scan type electrophotographic recording apparatus of the kind which uses minute light emitting segments as a light source.
Generally, in a laser printer or like optical scan type electrophotographic recording apparatus, a light beam modulated by binary video data is manipulated to sequentially expose a photoconductive element to form a latent image electrostatically thereon. The problem encountered with this kind of image recording is that because the pulse width of video data per pixel and, therefore, the ratio of a light beam exposing time to a one-pixel scanning time is fixed, the latent image potential in an image portion which borders a non-image portion is lowered due to building and falling of the latent image potential. Such causes the image to be developed to have different pixel diameters in the main scan and subscan directions and, thereby, considerably lowers the resolution. Especially, when it comes to document images, characters are prevented from appearing clear-cut.
Japanese Patent Laid-Open Publication No. 56-8112/1981 discloses an implementation for an optical scan type electrophotographic recording apparatus which modulates the pulse width of video data in order to eliminate thinning of images which is apt to occur during positive-to-positive recording, which develops unexposed portions. However, it fails to improve the quality of recoded images when a countermeasure is provided against thinning in the main scan direction only. That is, it cannot offer a desirable image quality unless a countermeasure covering both the main scan and subscan directions is provided in due consideration of gitter occurring on a photoconductive element, developing method, developing characteristics, etc. In that case, reproducibility on a one-dot line basis is very important.
Meanwhile, the countermeasure against thinning and a measure for an improvement in resolution, which the present invention contemplates, are contradictory to each other; the anti-thinning measure is not always advantageous in enhancing or stabilizing the image quality. That is, whether the recording be negative-to-positive which develops exposed portions or positive-to-negative which develops unexposed portions, attaching importance to the resolution rather than anti-thinning is advantageous from the viewpoint of improvement or stabilization of the image quality.
An optical scan type electrophotographic recording apparatus has been proposed which is elaborated to free a laser printer from the intricacy of construction of its optical system (Japanese Patent Laid-Open Publication No. 58-108864) and other problems. In the proposed apparatus, to simplify a light source and an optical system, a light source comprises a phosphor dot array tube having an array of phosphor elements arranged in the main scan direction in correspondence with pixels. Light issuing from the phosphor dot tube and modulated by binary video data is passed through an imaging system toward a photoconductive element, which is fed in the subscan direction, to provide a latent image thereon, the latent image being developed to record data associated with the video data.
However, due to the use of minute, light emitting phosphor elements as a light source, the above-described prior art method limits the available potential of latent images and, thereby, electrostatic contrast. This is apt to cause the resolution in the subscan direction to fluctuate due to gitter on the surface of the photoconductive element, greatly effecting the quality of the entire recorded images.
In addition, the dot array tube type scheme cannot accomplish a sufficient resolution, particularly sufficient reproducibility of hairlines such as one-dot lines. Specifically, because with respect to the main scan direction the intensity distribution of the light issuing from the regularly arranged light emitting elements has great influence and because, with respect to the subscan direction, the light emitting elements emit light at a predetermined timing associated with the movement of the photoconductive element in the subscan direction, in the case of recording hairlines such as one-dot lines, the width of the lines to be developed differs from the main scan direction to the subscan direction thereby deteriorating the resolution.